The present invention relates to a ventricular-assist method and apparatus and, more particularly, to a ventricular-assist device (VAD) which can assist especially a failing heart and delay the development of end-stage heart failure and the point at which a heart transplant may be required. The invention also relates to a method of sustaining the failing heart utilizing a ventricular-assist device and an algorithm for operating a ventricular-assist device.
The normal cardiac output, normalized by total body surface, is around 3.5 liter per minute per one square meter (1/min/m2). In general, cardiac assist is necessary whenever a patient""s cardiac output drops below the adequate blood supply needed to sustain proper blood perfusion, which is 1.8-23 1/min/m2. Failure to supply adequate flow is defined as xe2x80x9csystolic failurexe2x80x9d. However, more than 50% of the patients over 60 display inadequate ventricle filling and tissue congestion, which is defined as xe2x80x9cdiastolic failurexe2x80x9d. Cardiac assist is used to treat patients suffering from heart failure at a stage where conventional drug therapy proves ineffective.
Congestive Heart Failure (CHF) is a chronic disorder that develops over time, manifested clinically by an enlarged heart and symptoms and signs of low cardiac output and tissue congestion. The low cardiac output leads to decreased blood perfusion to vital organs (liver, kidney and brain). The CHF is also characterized by lung congestion (recurrent pulmonary edema) which threatens life and requires hospitalization. CHF is associated with profound symptoms that limit daily activities, is a debilitating disease with poor quality of life. CHF is the most common cause of hospitalization of patients over 60 years of age.
CHF has various etiologies, including cardiovascular disease (diseases which affect blood flow to the myocard), chronic hypertension (high blood pressure), incompetent values, inflammation of the heart muscle or the valves, substance accumulation (amyloid) and congenital heart problems.
Cardiovascular diseases (CVD) represent the leading cause of death in the industrialized world. CVD claimed 960,592 lives in the US in 1995 (41.5% of all deaths for that year). According to the US National Heart Lung and Blood Institute (NHLBI) and the American Heart Association there are approximately 5 million patients who suffer from Congestive Heart Failure (CHF) in the US and between 400,000 and 500,000 newly diagnosed patients each year. Long-term survival rates are low and the 5 year mortality rate for patients with CHF is 75% in men and 62% in women, while in patients with decompensated heart failure the mortality rate is 60% per year.
Patients suffering from Congestive Heart Failure (CHF) are initially treated with medication. While conventional drug therapy may delay the progress of CHF, it is not curative. Cardiologic intervention (such as Angioplasty and Stenting), surgery (Heart by-pass surgery, Cardiomyoplasty, Partial Ventriculectomy known as Batista""s procedure), and mechanical devices are often considered when drug therapies prove ineffective or inadequate. Electrical disturbances of the heart that threaten or impair the quality of the patient""s life have been treated effectively with pacemakers and implantable defibrillators. However, congestive heart failure has not been addressed effectively. Currently, the only available method of treating end-stage CHF is a heart transplant.
The demand for temporary and permanent cardiac-assist devices is remarkably large; in 1993 between 40,000 to 70,000 patients needed life-sustaining assist devices or a total artificial heart, and an additional 80,000 to 200,00 patients needed quality of life improvements by surgery (Cardiomyoplasty or Heart Booster).
Ventricular-assist devices are needed for:
Bridge-to-Recoveryxe2x80x94cardiac assist for patients whose heart has sustained serious injury, but can recover if adequately supported. This includes the use of a cardiac-assist device after surgery in order to provide support until the heart regains its ability to pump. Temporary cardiac support is intended primarily to prevent or reduce damage from cardiac failure or to support adequate blood circulation.
Bridge-to-Transplantationxe2x80x94patients awaiting heart transplants and who are not scheduled and when the heart failure is unresponsive to medical treatment.
Existing temporary mechanical cardiac devices are divided into three groups:
1. Temporary cardiac assist for several hours, as the intra-aortic balloon that is frequently utilized for patients with heart failure after open-heart surgery due to failure to wean from the cardiopulmonary bypass.
2. Long-term (days, weeks, months) Ventricular Assist Device (VAD), as a bridge to heart transplantation.
3. Permanent support by Total Artificial Heart (TAH).
Intra Aortic Balloon Pump (ABP). The IABP has been in clinical use for over 20 years. The IABP consists of a balloon (30-50 ml) that is inserted into the descending aorta and is inflated during the diastole and deflated during the systole. The IAB increases the cardiac output by less than 0.5 1/min/m2.
Consequently, although it was designed to assist a failing heart by improving blood perfusion, it requires a certain threshold level of cardiac output and cannot take over the pumping function of the heart. As a result, it can only be utilized in treatment of patients who require mild levels of mechanical assistance (unless there is a supplemental assisting heart device). The device reduces the energy consumption and allows the heart to recover. However, the IABP is used only for short-term circulatory assist due to high risk of severe thromboembolic complications.
Ventricular Assist Devices (VAD)xe2x80x94VADs take over the complete pumping function of one or both sides of a failing heart. They unload the assisted ventricle. Left Ventricular Assist Devices have been approved for use by the FDA as bridge-to-heart transplantation, to keep patients alive who are awaiting a donor heart. These devices have also been approved for use by patients whose hearts are in failure but may be able to recover by reducing the myocardial work (unloading), including patients in post-surgical life-threatening heart failure.
More than a dozen companies (listed below) are developing devices, ranging from left-ventricular assist products to total artificial hearts, that offer CHF patients either longer-term support with an alleviation of symptoms, and/or an alternative to heart transplant. Some of these (Thermo CardioSystems, Thortec, Abiomed and Baxter Healthcare) have ventricular assist products on the U.S. market. Ventricular-assist devices are generally employed on a temporary basis, with treatment periods ranging from a few hours to a few weeks, or at most, a limited number of months. However, some devices have been designed for long-term use and can be considered lifetime support systems. To date, such lifetime support is still in developmental and experimental stages and was not approved by the FDA.
There are five major types of VAD: Roller pumps, Centrifugal pumps, Pneumatic devices, Electrical devices and direct mechanical actuators. These devices differ in the design, indications and duration.
Roller and Centrifugal Pumps are approved for short-term (i.e. hours) support of patients undergoing heart surgery. These devices generate a non-pulsatile blood flow which severely restricts the time patients can safely remain on support. They also require additional medical personnel to provide constant monitoring and ensure that the pump is operating correctly.
Transplant bridging, and possibly long-term cardiac assistance may also be accomplished with implantable axial flow and centrifugal pumps. Examples of companies pursuing cardiac-pumping technology include: Jarvik Research, Medtronic Inc., 3M Corporation Inc., Kirton Medical, Micromed Technology and Cardiac Assist Technologies.
A high-speed pump has been developed recently by Micromed in co-development with the National Aeronautics and Space Administration (NASA). This miniaturized DeBakey/Ventricular Assist Device (30 mmxc3x9776 mm) weighs only 93 grams, making it about one-tenth the size of portable heart-assist devices already on the market.
The pneumatic devices were the first to be approved for clinical use. Through December 1997 the BVS 5000, developed and manufactured by Abiomed Inc. was the only approved product, and it is the only device that can provide full circulatory assistance approved by the US FDA as a bridge-to-recovery device for the treatment of reversible heart failure. The BVS-5000 (BVS) is a pneumatic extra-corporeal, bi-ventricular assist device, allowing the heart to rest, heat and recover its function. However, the blood circulates out of the body and the patient cannot be ambulatory. The company""s first full year of marketing the BVS in the U.S. was 1994.
Thoratec Laboratories Corporation has developed an implantable pneumatic-assist device, which is connected to an external drive by a percutaneous air-drive line. This system was also approved by the FDA as a bridge to heart transplant.
The electrical VAD will ultimately be completely implantable with an implantable controller, battery and charger (secondary coil). The main electrical pulsatile implantable pump are: Novacor N-100 (Baxter Healthcare Corp.), Heartmate 1000 NE LVAS (ThermoCardioSystem Inc.) and Pennsylvania State University System.
In September 1998, the first two ambulatory implantable left ventricular-assist systems (LVAS), from Baxter and ThermoCardioSystem Inc (TCS), were approved in the U.S. TCS"" implantable electric HeartMate LVAS has been marketed since 1994. In Europe, the Baxter Novacor LVAS has been approved as a commercial product since 1994. These devices represent a significant advance over first-generation technology, since they allow patients to live outside the hospital while awaiting transplantation. The Baxter Novacor is an electromechanical pump that is implanted in a patient""s abdomen and connected to the left ventricle of the heart. The system is operated by an external, portable electronic controller, and is powered by battery packs, which the patient typically wears around the waist in a shoulder vest or backpack. Nearly 900 patients worldwide have received the Novacor LVAS: two patients have currently been supported for more than three years by their original devices. In Europe, the device has helped to rehabilitate more than 20 patients"" hearts to the extent that neither VAD assistance, nor heart transplant is necessary.
The Direct Mechanical Actuator is a different approach, taken by Cardio Technologies. This company is pursuing a cuff-like device that is placed around the outside of the heart. This device applies external pressure to enhance blood flow. A somewhat similar device, designed to reduce the size of an enlarged heart, is under development by Acorn Cardiovascular. Abiomed is also in early development stages of the Heart Booster system designed to wrap around the heart to provide ventricular augmentation.
Three additional surgical methods have been developed recently as alternative to cardiac assist, in order to improve the residual cardiac function: 1) Dynamic Cardiomyoplasty; 2) Partial Ventriculectomy or Batista operation, and 3) Percutaneous transmyocardial revascularization (PTMR). However, these methods are controversial.
In the Dynamic Cardiomyoplasty technique, a surgeon wraps some of the patient""s skeletal muscle around the weakened heart and stimulates the repositioned muscle to synchronously squeeze the heart during diastole. Dynamic Cardiomyoplasty is highly invasive and involves complicated surgical procedures. Medtronic is also involved in clinical studies of this pacemaker-aided technique using the latissimus dorsi muscle.
Percutaneous transmyocardial revascularization (PTMR) is a recently approved catheter-based laser technique that involves drilling about 50 tiny holes in the left ventricle to improve blood flow to the heart muscle. The laser surgery offers a cost-effective alternative to transplantation for certain patients with severe angina, who were not candidates for angioplasty or bypass surgery. The precise mechanism underlying this approach is controversial.
It is the principal object of the present invention to provide an improved ventricular-assist device that is free from the drawbacks of earlier devices, is especially effective in assisting a failing heart and in some cases may even be able to improve the cardiac function of the natural tissue of a failing heart.
It is also an object of the invention to provide an improved method of assisting a failing heart.
Still a further object of the invention is to provide a method of and an apparatus for ventricular assistance whereby drawbacks of earlier systems can be avoided, the assistance provided can be more reliable and the energy drain on the assisted heart can be minimized.
These objects are attained, in accordance with the invention in a ventricular-assist method which comprises:
(a) inserting into the failing ventricular cavity (left, right or both) of a failing heart through a wall thereof a respective expandable intraventricular chamber having a maximum volume of 30 ml;
(b) in cadence with normal functioning of the failing heart, effecting expansion of the respective intraventricular chamber with each heart beat and commencing only after opening of an outlet valve of the respective ventricular cavity of the failing heart or only after a detected shortening of a monitored region of a wall of the respective ventricular cavity of the failing heart and continuing during an ejection phase of the respective ventricular cavity, thereby augmenting ejection volume from the respective ventricular cavity by up a maximum volume of the intraventricular chamber per systolic phase;
(c) controlling a time course of expansion of each intraventricular chamber in step (b) to reduce a shortening of a respective ventricular wall of the failing heart by comparison with ventricular wall shortening prior to insertion of the respective intraventricular; and
(d) depressurizing and contracting each the intraventricular chamber immediately upon closing of a respective outlet valve of the failing heart.
The method of the invention further comprises the steps of:
measuring ventricular wall motion during expansion of a respective intraventricular chamber in step (b); and
controlling a profile of the expanding intraventricular chamber in a course of expansion thereof to decrease the measured ventricular wall motion thereby obtaining an increase in the pressure within the respective cavity and increase the cardiac output.
According to the invention, at least one parameter of ventricular wall shortening and at least one parameter of ventricle output can be measured during systole and in response to measurement of these parameters, selectively either in real time or by beat-by-beat computation, and a designed shape for each interventricular chamber is determined and the shape and a rate of expansion and contraction of the interventricular chamber are controlled to correspond to the desired shape.
The parameters of wall shortening which can be monitored are the ventricular diameter, ventricular volume, and ventricular wall strain or the ventricular flow in preferred embodiments of the invention. The interventricular chamber is preferably a balloon with computer-controlled expansion and implanted in either interventricular cavity or both interventricular cavities.
In terms of the apparatus, the system can comprise the steps of:
(a) inserting a failing ventricular cavity (left, right or both) of a failing heart through a wall thereof a respective expandable intraventricular chamber having a maximum volume of 30 ml;
(b) in cadence with normal functioning of the failing heart, effecting expansion of the intraventricular chamber with each heart beat and commencing only after opening of an outlet valve of the respective ventricular cavity of the failing heart or only after a detected shortening of a monitored region of a wall of the respective ventricular cavity of the failing heart and continuing during an ejection phase of the respective ventricular cavity, thereby augmenting ejection volume from the respective ventricular cavity by up a maximum volume of the intraventricular chamber per systolic phase;
(c) controlling a course of expansion of each the intraventricular chamber in step (b) to reduce a shortening of a respective ventricular wall of the failing heart by comparison with ventricular wall shortening prior to insertion of the respective intraventricular; and
(d) depressurizing and contracting each the intraventricular chamber immediately upon closing of a respective outlet valve of the failing heart.
The apparatus can have a computer receiving input from the sensor and controlling the actuator with an output, the computer being programmed for each heartbeat (n) to:
(a) evaluate cardiac output and work at the n beat;
(b) compare the evaluated cardiac output and work at the n beat with a desired cardiac output to determine an amplification factor (AF);
(c) multiplying the amplification factor (AF) by a weighting function (W(t)) as determined by an operator to generate a magnitude of a feedback loop;
(d) evaluate ventricle wall shortening (Sn(t)) and compare the evaluated wall shortening with a desired wall shortening (Des(t)) to obtain a difference Errn(t)=Des(t)xe2x88x92Sn(t);
(e) generate an expansion function EXPn+1(t)=EXPn(t)+AF*W(t)*Errn(t); and
(f) control expansion of the intraventricular chamber at a next beat (n+1) with the expansion function EXPn+1(t)=EXPn(t)+AF*W(t)*Errn(t).
Advantageously the computer is a computer which controls the intraventricular chamber at the next beat (n+1) with the expansion function by regulating onset time of expansion and the function of expansion.
The amplification factor (AF) is multiplied by a weighting factor (W(t)) at each beat where 0xe2x89xa6W(t)xe2x89xa61 and 0xe2x89xa6txe2x89xa6T, and t=0 is the onset of expansion and t=T is the end of expansion.
The computer can receive input from the sensor and can control the actuator with an output, the computer being programmed for each heartbeat (n) to:
(a) evaluate cardiac output and work at the n beat;
(b) compare the evaluated cardiac output and work at the n beat with a desired cardiac output and determine an amplification factor that will not cause wall stretch in part based upon additional inputs;
(c) evaluate ventricle wall shortening (Sn(t)) at the n beat and providing the ventricle wall shortening as one of the additional inputs;
(d) detect possible ventricle wall lengthening from the evaluation of the wall shortening in step (c) and providing therewith another of the additional inputs, and triggering an alarm upon ventricular wall lengthening
(f) from the amplification factor and a desired profile of expansion, determine a time course of expansion of the intraventricular chamber; and
(g) generate an expansion function representing the time course of expansion control of the intraventricular chamber at a next beat (n+1) with the expansion function.
The means for detecting a state of the outlet valve can include at least one of the following:
means for measuring intraventricular and aortic pressure or a gradient between intraventricular and aortic pressure;
a Doppler or an ultrasonic or electromagnetic flowmeter measuring ventricle outlet flow;
ultrasound or electrical impedance means for measuring intraventricular volume;
strain gauge means for measuring ventricle wall shortening; and
means for detecting heart sounds.